Dimers of enkephalins using methylene crosslinking moieties were used as probes of the properties of opioid receptors in brain and cultured cells (NG108-15 cells). A new series of tripeptide dimers (designated DTRE) has been studied. The monomer is extremely weak and mu selective. The dimers have 200-400 fold increased potency. Short-chain dimers are mu selective; long-chain dimers are delta selective. A new series of alkyl-amide derivatives of [D-Ala,2Leu5]enkephalinamide has been studied, and indicates the extent to which bivalency is responsible for the increased potency and specificity. Kinetic studies indicate that 1) dimers have exactly half the molar binding capacity of monomers; 2) dissociation is not accelerated by the presence of unlabeled ligand; 3) reduction of receptor density by an affinity label does not appear to selectively reduce the affinity for the dimer. Collectively, these studies suggest a new model for interaction between the dimer and the receptor, involving binding to two sites within the same receptor.